Add doctests for `label` templatetag
Update factories

Break down factories in to smaller components. For example, class UserFactory
now populates its `username` field using the `user_username()` function, and
class ItemFactory populates its `name` field using the `item_name()` function.
This change has two benefits:

* The functions that generate values for individual fields can be tested
  individually. Formerly, the factory as a whole could be tested, but not
  individual fields.
* Code in other modules can now use these functions. `test_forms.py` in
  particular will benefit from this change.
Update unit tests.

Update the unit tests in `test_forms.py` and `test_views.py`. These unit tests
are now much more stringent. They also take advantage of the recent changes to
`factories.py`, which allows them to be much more stringent.
Satisfy unit tests

Make `forms.py` satisfy unit tests.